In a microprocessor controlled apparatus a data bus is often employed for communication. Usually, since the data source, data recipient and bus are all contained within the apparatus little consideration is given to the techniques employed when data transmission extends beyond the equipment chassis. Many electronic systems facilitate remotely controlled activation and operation. Clearly remote activation requires that some part of the controllable apparatus be on and dissipating power in order to receive the remote command. Hence, in the interest of power conservation, most electronic systems employ two operational conditions namely a STANDBY mode and RUN or OPERATE mode. Effectively two power supplies are provided, a STANDBY supply which is permanently energized while connected to an AC mains supply, and a RUN or OPERATE supply which is turned on following user activation.
In the exemplary apparatus described above, the use of standby and operate modes with their respective power supplies may result in problems occurring between circuitry with power sustained, and circuitry where the power is switched. For example, a data bus may originate from a power sustained microprocessor to provide communication to circuitry powered only during an operate mode. Typically such bus controlled circuitry forms part of an integrated circuit which may include electrostatic discharge protection for any input pins. Typically electrostatic or ESD protection may be provided by a series connected diode structure with the cathode connected to the IC power supply Vcc and the anode connected to the lowest circuit potential, typically ground. The junction of the two diodes is connected to the IC input pin. A diode conducts whenever the input pin potential exceeds the potentials applied to the respective diode ends.
Hence in a system where a data bus originates from a power sustained microprocessor and is coupled to power switched integrated circuit with input pin protection a problem results where the data bus is clamped, usually to the absent switched Vcc supply. The data bus clamping results from current flow between a data bus pull up resistor and an ESD protection diode at the input pin of the IC. Thus the data bus is inoperative and the apparatus is incapable of either remote or local activation.